MECHANICAL SEALS AND METHODS OF MAKING
BACKGROUND
The present disclosure relates to seals, and more particularly to mechanical seals for
use in rotating machinery.
Mechanical seals are used in a variety of rotary shaft devices including blowers,
compressors, vacuum pumps, expanders, hot gas path assemblies, and the like. These
seals minimize or prevent fluid (either gas or liquid) from escaping a working
chamber containing the rotating shaft by providing a barrier e.g., between the working
chamber and an external environment or between the two consecutive stages of a
compressor or turbine.
One such type of mechanical face seal is a spiral groove seal, in which spiral-shaped
groove areas are provided on one of a pair of opposing seal faces. Upon rotation of
one of the seal faces relative to the other, due to hydrodynamic action, fluid is forced
through the grooves toward a non-grooved portion of the seal face. At a certain
speed, depending on the design of the seal, the fluid pressure, owing to this pumping
action, will separate the seal faces by a precise amount. The non-grooved portion of
the seal face serves as a seal dam that provides resistance to fluid escape and also
maintains uniform fluid pressure. One of the pair of opposing seal faces may be
spring loaded to provide additional resistance to fluid escape by ensuring that a force
is applied to oppose separation of the two seal faces and to minimize the gap between
the seal faces.
When the rotation speed is too low (e.g., during start up and shut down of the device
in which the spiral groove seal is installed), there is not enough pressure generated to
separate the sealing faces. As a result, contact between the seal faces occurs which,
even though brief, may be sufficient to generate microcracks, grain pull out, and/or
grain disintegration at the seal faces owing to frictional heat and wear. The
concentration of these defects can increase from repeated contact events and/or from
the hoop and centrifugal stresses that are generated during normal operating
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conditions (i.e., rotation speeds), leading to failure of the seal and ultimately failure of
the seal housing and/or device employing the seal.
Previous efforts to increase the lifetime of a seal have focused on increasing hardness,
increasing crack resistance, decreasing friction, minimizing the number of contact
events, and the like. Many seal faces are now formed from high performance carbides
(e.g., tungsten carbide, silicon carbide, and the like, in their various forms) instead of
oxides or metals. However, many of these seals are limited in thickness and,
therefore, cannot sustain the wear conditions to which they are exposed during their
operational lifetime. Accordingly, despite the improvements that have been made,
there nonetheless remains a need in the art for improved mechanical seals.
BRIEF SUMMARY
A mechanical seal includes a pair of opposing seal faces, wherein at least one of the
pair of seal faces comprises a multilayer coating disposed on a substrate, and wherein
the multilayer coating comprises a periodic repetition of distinct layers.
In another embodiment, the mechanical seal includes a pair of opposing seal faces,
wherein at least one of the pair of seal faces comprises a multilayer coating disposed
on a substrate, wherein the multilayer coating comprises a plurality of layers of a
composite, and wherein no two adjacent layers of the composite comprise an identical
ratio of composite constituents.
A method includes disposing a multilayer coating on a substrate to form at least one
of a pair of opposing seal faces of a noncontacting mechanical seal.
The above described and other features are exemplified by the following figures and
detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the figures, which are exemplary embodiments and wherein like
elements are numbered alike:
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Figure 1 1s a longitudinal cross-sectional representation of a portion of a
noncontacting mechanical seal face; and
Figure 2 is a schematic illustration of a noncontacting mechanical seal face having
spiral shaped grooves.
DETAILED DESCRIPTION
Mechanical seals and their methods of manufacture are disclosed herein. In
exemplary embodiments, the mechanical seals are noncontacting mechanical seals.
The term "noncontacting" when used herein to describe a seal has its art recognized
meaning (i.e., that there is a pressure-generated separation between opposing seal
faces at some point during the operation of the device employing the seal). The
rotational speed at which the separation between opposing seal faces occurs is also a
function of the surface finish of the seal faces. Over time, upon degradation of a seal
surface, higher rotational speeds are needed for separation, resulting in increased
generation of heat and wear on the surfaces of the seal faces while they are in contact.
In contrast to the prior art, the seals and methods disclosed herein are generally based
on at least one of the seal faces comprising a multilayer coating. The use of the
multilayer coating advantageously results in a hard wear resistant seal face having
reduced friction and reduced wear related microcrack formation. These features
ultimately result in increased seal and device lifetimes.
Also, the terms "first", "second", and the like do not denote any order, quantity, or
importance, but rather are used to distinguish one element from another; and the terms
"the", "a", and "an" do not denote a limitation of quantity, but rather denote the
presence of at least one of the referenced items. The modifier "about" used in
connection with a quantity is inclusive of the stated value and has the meaning
dictated by the context, (e.g., includes the degree of error associated with
measurement of the particular quantity). Furthermore, all ranges reciting the same
quantity or physical property are inclusive of the recited endpoints and independently
combinable.
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The seal generally includes a pair of opposing seal faces, wherein at least one seal
face comprises a multilayer coating disposed on a substrate. During operation of the
seal, one of the seal faces rotates with respect to the other. While either (or both) seal
face may comprise the multilayer coating, it is desirable that at least the rotating seal
face comprises the multilayer coating. Furthermore, either (or both) seal face may
optionally comprise spiral-shaped grooves; however, it is desirable that at least the
rotating seal face has these grooves.
Referring now to Figure 1, a portion of a seal face, designated 10, is illustrated. The
portion of the seal face 10 generally includes the substrate 12 and the multilayer
coating 14 disposed thereon.
The substrate 12 onto which the multilayer coating 14 is disposed may be any metal,
metallic alloy, or ceramic (e.g., oxide, nitride, carbide, and the like) composition. In
an exemplary embodiment, the substrate 12 is a carbide composition. Exemplary
carbides include silicon carbide (e.g., solid silicon carbide, siliconized graphite,
reaction bonded silicon carbide, self-sintered silicon carbide, or a composite
comprising at least one of the foregoing) and tungsten carbide (e.g., tungsten carbide
or a metal-bonded tungsten carbide). It is important to note that the composition and
the microstructure of the substrate can affect the performance of the seal face.
Within the multilayer coating 14, the composition of each layer may be chosen to
provide a desired property such as hardness, wear resistance, lubricity, them1al stress
resistance, fracture toughness, adherence, or a combination comprising at least one of
the foregoing properties.
By way of example, when hardness, wear resistance, and/or them1al stress resistance
are desired, a ceramic material may be used as a composition for a layer of the
multilayer coating 14. Suitable ceramic compositions include hard phase metal
oxides such as Ah03, Cr203, Zr02, and the like; metal carbides such as Cr3C2, WC,
TiC, ZrC, B4C, and the like; diamond, diamond-like carbon; metal nitrides such as
cubic BN, TiN, ZrN, HfN, Si3N4 , AlN, TiAlN, TiAlCrN, TiCrN, TiZrN, and the like;
metal borides such as TiB2, ZrB2, Cr3B2, W 2B2, and the like; and combinations
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comprising at least one of the foregoing compositions. Alternatively, a composition
for a layer of the multilayer coating 14 is a ceramic composite comprising at least 51
volume percent (vol %), based on the total volume of the composite, of the
aforedescribed suitable ceramic compositions and a binder phase of a relatively soft
and low melting composition. Suitable ceramic binder phase compositions for the
ceramic composite include Si02, Ce02, Y203, Ti02, and combinations comprising at
least one of the foregoing ceramic binder phase compositions. In yet another
alternative, a composition for a layer of the multilayer coating 14 is a ceramic-metal
composite (cermet). Suitable cermets include WC/Co, WC/CoCr, WC/Ni, TiC/Ni,
TiC/Fe, Ni(Cr)/Cr3C2, and combinations comprising at least one of the foregoing.
Still other compositions for a layer of the multilayer coating 14 include combinations
comprising at least one of the ceramic, ceramic composites, or cermets (e.g., a metal
or alloy matrix comprising one of the foregoing).
In another example, when lubricity is desired, a composition for a layer of the
multilayer coating 14 may comprise germanium, MoS2, a polyamide, a fluoropolymer
(e.g., polytetrafluoroethylene, fluorinated ethylenepolypropylene, or the like),
graphite, a transition metal boride, hexagonal boron nitride, and like solid lubricants.
Advantageously, these solid lubricants, when in the form of a powder, in addition to
providing lubricity, will also facilitate the removal of heat from the contact area of the
seal.
In another example, where adherence is desired, a composition for a layer of the
multilayer coating 14 may comprise a MCrAl or MCrAlY alloy, wherein M
represents a metal such as iron, nickel, or cobalt; NiAl(Zr) compositions; and the like.
While there is no specific upper limit to the number of individual layers that may
form the multilayer coating 14, there must be at least two 2 layers. Within the
multilayer coating 14, the thermal expansion of the individual layers with the
substrate and between the individual layers should be considered. Additionally, the
layers should be able to tolerate any non-uniform strain.
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Furthermore, within the multilayer coating 14, each layer may have a different
thickness and/or each layer may have a non-uniform thickness. The average thickness
of each layer may independently be about 5 nanometers (nm) to about 25 micrometers
(IJ.m). Within this range, the average thickness of each layer can independently be
greater than or equal to about I 0 nm, specifically greater than or equal to about 20
nm. Also within this range, the average thickness of each layer can independently be
less than or equal to 10 IJ.m, specifically less than or equal to about 5 !J.ffi. The
average thickness of the overall multilayer coating 14 may be about 2 IJ.m to about
500 !J.ffi. Within this range, the average thickness of the overall multilayer coating 14
can be greater than or equal to about 5 IJ.m, specifically greater than or equal to about
8 IJ.m. Also within this range, the average thickness of the thickness of the overall
multilayer coating 14 can be less than or equal to 200 IJ.m, specifically less than or
equal to about 50 IJ.m.
In one embodiment, at least a portion of the multilayer coating 14 can be a periodic
repetition of individual layers. For example, two different compositions can be
altematingly stacked to form 3 or more layers. In addition, 3 different compositions
may be stacked in any number of permutations including, but not limited to, 1-2-3-1-
2-3-, 1-2-3-2-1-, and the like. If these altematingly stacked layers are sufficiently thin
(e.g., les$ than or equal to about 100 nm), a heterostructure or superlattice is formed,
which can have a significantly improved hardness and fracture resistance than a
thicker, individual layer.
In another embodiment, the multilayer coating 14 can comprise more than one
individual layer of a composite such that each layer has the same constituent
components but in different amounts or ratios. For example, a composite comprising
nanoparticles dispersed in a matrix can be used for each layer of the multilayer
coating 14 with increasing (or decreasing) amounts of the nanoparticles in the next
adjacent layer such that a gradient in properties exists. As used herein, in reference to
layers of the multilayer coating, the term "adjacent" refers to two layers that are in
physical contact (i.e., there is no intervening layer disposed between two layers
referred to as being adjacent to one another).
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Each layer of the multilayer coating 14 can independently be deposited or otherwise
formed on the substrate 12 by any of a variety of suitable techniques, such as physical
vapor deposition (PVD), including electron beam physical vapor deposition (EBPVD),
radio frequency sputtering, ion beam sputtering, plasma assisted physical
vapor deposition, cathodic arc deposition, and cathodic arc ion plasma deposition;
chemical vapor deposition (eVD); and the like. Each of these techniques can be used
to form the individual layers of the multilayer coating 14 by those skilled in the art in
view of this disclosure without undue experimentation. Depending on the technique
used, the atomic structure of each layer can independently be tailored to be crystalline
or amorphous as may be desired for a particular seal application. Furthermore, if
crystalline, the grain morphology can also be tailored as desired for the particular seal
application.
Referring again to Figure 1, an exemplary rotating seal face 10 of a mechanical seal
can be formed on a Ni-bonded we cermet substrate 12. The average longest
dimension of a grain of We is greater than or equal to about 3 !Jlll. Specifically, the
grain size distribution is bimodal, wherein about 60 % of the grains have an average
longest dimension greater than about 3 !Jlll and the balance of the grains are have an
average longest dimension less than about 2 Jlm. The amount of nickel present in the
cermet is about 6 to about 15 weight percent ( wt %) based on the total weight of the
cermet. Within this range, it is desirable to have greater than or equal to about 9 wt %
nickel present in the cermet. In this manner, any cracks that are generated in the we
are prevented from growing by the presence of an increased amount of nickel.
The multilayer coating 14 is formed by depositing alternating layers of TiN (18, 22,
and 26) and ZrN (20 and 24). It should be recognized that while reference has been
made to 5 alternating layers (i.e., 18, 20, 22, 24, and 26), this is only for illustrative
purposes. One of ordinary skill in the art will appreciate that any number of
alternating layers may be used. Furthermore, although the first alternating layer 18
(i.e., the layer closest to the substrate) in this embodiment has been refen·ed to as a
TiN layer, it is possible for ZrN to be used as the first alternating layer 18.
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Alternating layers 18, 20, 22, 24, and 26 are deposited by a PVD technique, and can
be directly deposited onto the substrate 12 or onto an optional adhering layer 16,
which may better adhere to the substrate 12 than the first alternating layer 18. The
optional adhering layer 16 can be deposited using CVD so as to provide better control
over the grain growth on the substrate. It is desirable for the optional adhering layer
16 to have the same composition as the first alternating layer 18 to provide the
greatest compatibility therebetween. The alternating layers 18, 20, 22, 24, and 26
generally each have a thickness of about 20 nm to about 100 nm so as to form a
heterostructure. In one embodiment, the cumulative thickness of all of the alternating
layers 18, 20, 22, 24, and 26 is about 3 ).!ill to about 8 ).!ill. The optional adhering
layer 16 can have a thickness of about 1 ).!ill to about 25 ).!ill.
Optionally, a low friction layer 28 may be disposed on the last (i.e., outermost from
the substrate 12) alternating layer 26. The optional low friction layer 28 can be, for
example, a diamond-like carbon layer. Any of the above-described techniques may
be used to deposit the optional low friction layer 28.
Alternatively, or in addition to the optional low friction layer 28, an optional solid
lubricant layer 30 may be disposed on the last alternating layer 26 (or the optional low
friction layer 28) to provide increasing lubricity to the rotating seal face 10 when it
contacts the opposing seal face (not shown). The optional solid lubricant layer 30
may be burnished or deposited on the l~st alternating layer 26 (or the optional low
friction layer 28) using a binder phase.
Once the uppermost layer of the multilayer coating 14 has been deposited onto the
substrate 12, spiral-shaped grooves 32, as shown in Figure 2, may be etched or
machined into the surface of the uppermost layer. It is also possible for the substrate
12 to be machined or etched prior to deposition of the multilayer coating 14, while
maintaining the spiral-shaped grooves 32 after deposition. Exemplary techniques for
depositing the multilayer coating 14 on an already machined or etched substrate 12
include EB-PVD, cathodic arc deposition, and the like.
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In another exemplary embodiment, instead of alternating layers of TiN (18, 22, and
26) and ZrN (20 and 24), a plurality of layers of a composite comprising Ah03
nanoparticles dispersed in a nanostructured NiCrAl or CoCrAl alloy matrix are used
such that each layer in the plurality of layers has a different concentration (e.g.,
volume fraction) of Ah03 nanoparticles than any immediately adjacent layer. For
example, one layer may be ceramic-rich, while the next adjacent layer may be alloy
rich. Within the composite, the Ah03 nanoparticles have an average longest
dimension of about 10 nm to about 200 nm. Also within the cumulative composite,
the overall Alz03 volume fraction is high (e.g., about 70 to about 80 %) so as to
provide increased hardness to the composite.
The plurality of composite layers ( 18, 20, 22, 24, and 26) are deposited by a PVD
technique and, similarly, can be directly deposited onto the substrate 12 or onto the
optional adhering layer 16, which may better adhere to the substrate 12 than the first
composite layer 18. The overall thickness ofthe plurality of composite layers (18, 20,
22, 24, and 26) is about 50 Jlm to about 300 Jlm.
In yet another exemplary embodiment, the multilayer coating 14 compnses a
combination of the plurality of composite layers and the heterostructure to provide
still increased hardness. Desirably, the plurality of composite layers is deposited on
the substrate prior to the layers comprising the heterostructure such that the
heterostructure is disposed on the plurality of composite layers on a surface opposite
the substrate.
It should be recognized by those skilled in the art that the mechanical seals and
methods disclosed herein provide hard wear resistant seal faces having reduced
friction and reduced wear related microcrack formation. For example, the Vickers
hardness (Hv) of a seal face comprising the multilayered coating may be about 4000 to
about 5000. As a result, the lifetimes of the mechanical seal and the device
employing the seal may be significantly improved.
It should also be recognized that the mechanical seal and the device employing the
seal may include other components that are known to be used with mechanical seals
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and devices employing mechanical seals, such as springs (for spring-loading at least
one of the pair of spring faces, which is generally the static face), shafts, rotors,
stators, secondary o-ring seals, and the like.
While the disclosure has been described with reference to exemplary embodiments, it
will be understood by those skilled in the art that various changes may be made and
equivalents may be substituted for elements thereof without departing from the scope
of the disclosure. In addition, many modifications may be made to adapt a particular
situation or material to the teachings of the disclosure without departing from the
essential scope thereof. Therefore, it is intended that the disclosure not be limited to
the particular embodiment disclosed as the best mode contemplated for carrying out
this disclosure, but that the disclosure will include all embodiments falling within the
scope of the appended claims.

WHAT IS CLAIMED IS:
1. A mechanical seal, comprising:
a pair of opposing seal faces (10), wherein at least one of the pair of seal faces (10)
comprises a multilayer coating (14) disposed on a substrate (12), wherein the
multilayer coating (14) comprises a periodic repetition of distinct layers, a plurality of
layers of a composite wherein no two adjacent layers of the composite comprise an
identical ratio of composite constituents, or both.
2. The mechanical seal of Claim 1, wherein the distinct layers form a
heterostructure.
3. The mechanical seal of any of the preceding Claims, wherein the multilayer
coating (14) further comprises an adhering layer (16), a low friction layer (28), a
lubricant layer (30), or combinations thereof.
4. The mechanical seal of any of the preceding Claims, wherein the at least one
of the pair of seal faces (10) comprising the multilayer coating (14) disposed on the
substrate (12) rotates during operation of the noncontacting mechanical seal.
5. The mechanical seal of any of the preceding Claims, wherein the at least one
of the pair of seal faces (10) comprising the multilayer coating (14) disposed on the
substrate (12) comprises spiral-shaped grooves (32).
6. The mechanical seal of any of the preceding Claims, wherein the mechanical
seal is a noncontacting mechanical seal.
7. A method, comprising:
disposing a multilayer coating (14) on a substrate (12) to form at least one of a pair of
opposing seal faces (10) of a mechanical seal.
8. The method of Claim 7, wherein the multilayer coating (14) compnses a
periodic repetition of distinct layers, a plurality of layers of a composite wherein no
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two adjacent layers of the composite compnse an identical ratio of composite
constituents, or both.
9. The method of Claim 8, wherein any of the layers of the multilayer coating
(14) are disposed using physical vapor deposition.
10. The method of Claim 7, further comprising chemical vapor depositing an
adhering layer (16) on the substrate (12) prior to disposing the multilayer coating (14)
on the substrate (12).